The present invention relates generally to a composite interface pad or coating for use in combination with heat generating semiconductor devices, and more particularly to a composite conformable member consisting of a base thermally conductive layer having an integral and compatible anti-blocking layer (hereinafter sometimes referred to as xe2x80x9clacquerxe2x80x9d) formed on an outer surface thereof. In addition to the anti-blocking properties which are important to a user prior to placement of the material in an operative assembly, this outer non-tacky release layer serves to provide significant protection for the base component against surface contamination. In addition, the anti-blocking properties may allow the electronic assembly to be xe2x80x9creworkedxe2x80x9dxe2x80x94i.e. disassembled and reassembled without the need to change the thermal interface pad. At the same time, the thermal and mechanical properties of the underlying bulk layer are not adversely affected, and remain present in the hard coated composite product. In use, the conformable composite is positioned between the opposed surfaces of a semiconductor device and a chassis or heat spreader, and accordingly becomes subjected to the elevated temperatures created and experienced by the semiconductor device during its normal operation and function.
The conformable composite member with its release surface nevertheless serves to enhance heat transfer through the elimination of voids or entrapped air which may otherwise be present along any irregular configurations formed in the opposed surfaces of the semiconductor device and chassis or heat spreader. The anti-blocking layer on the surface of the thermally conductive base layer eliminates the need for a liner film of the type which has heretofore been utilized as a protective layer and to accomplish or assist in anti-blocking. Thus, use of the conformable composite of the present invention serves to eliminate one or more significant labor-intensive operations, namely the elimination of the need to physically remove the liner film from the surface of a conformable thermally interface conductive member. Because the release surface layer is quite thin, it does not detract from the ability of the entire pad or layer to conform to the configuration of the surfaces of the members between which it is interposed.
Once it has served its purpose, the liner film must be removed prior to the point in time when it is placed into contact with the semiconductor device. This removal operation has frequently proven to be bothersome, and always time consuming and labor intensive. Hence this becomes a significant factor in the overall cost of assembly. The features of the present invention eliminate the need for the liner film, while at the same time the integrity and thermal properties of the thermally conductive composite are preserved. Depending upon the intended application for the composite, it may nevertheless be necessary to apply a liner film or tape to one surface, but not to both. Normally, when two liner films are employed, they are each removed at different points in time. In this connection, when a single liner film is used, the thin anti-blocking layer will be applied to the surface without the liner film or tape.
The present invention may also provide the ability for the electronic assembly to be disassembled and repaired without the need for replacing the phase change interface pad, since the anti-blocking layer will allow heat transfer surfaces to separate cleanly and to be reassembled.
The base or bulk portion of the composite interface upon which the release surface is applied is preferably of the phase-change type. The formulation normally comprises a blend of a microcrystalline wax, with alumina particulate being added to the wax to enhance thermal conductivity. Silicone wax may be added to the formulation for the base portion, if desired. The microcrystalline wax preferably has a melting point in the range of between about 40xc2x0 C. and 80xc2x0 C. In order to improve the wettability of the alumina particulate in the wax matrix, a quantity of an alkyl silane, such as for example octyl- or methyl-triethoxy silane may be utilized. This formulation is then employed to form the phase-change component forming the base or bulk portion of the composite. Other formulations useful in creating the base portion of the interface include those disclosed in application Ser. No. 09/016,768, filed Jan. 30, 1998, entitled xe2x80x9cTHERMAL INTERFACES FOR ELECTRONIC DEVICESxe2x80x9d, as well as in application Ser. No. 08/663,800, filed Jun. 14, 1996, entitled xe2x80x9cSEMISOLID THERMAL INTERFACE WITH LOW FLOW RESISTANCExe2x80x9d, now U.S. Pat. No. 5,950,066, both of which application and patent are assigned to the assignee of the present invention. Their disclosures are incorporated herein by reference.
The phase change portion of the conformable composite becomes at least partially liquid at temperatures encountered during normal operation of the solid state or semiconductor device with which it is being used. The phase change conformable composites of the present invention provide a highly thermally conductive path for dissipation of thermal energy and are effective independent of their immediate phase. The conformable composites find application on the surfaces upon which the semiconductor device is either being directly mounted, or alternatively, to which the device is being otherwise operatively thermally coupled. They may be applied as required onto surfaces of either metallic or polymeric materials. These conformable composites are preferably die-cut, although other operations may be employed.
Solid state electronic devices or components are incorporated in electronic systems generally, including systems used in the fields of data processing, communications, power supply systems, among others. Solid state electronic devices including power transistors, power modules including converts such as AC-to-DC and DC-to-DC and other similar components. The terms xe2x80x9csemiconductor devicexe2x80x9d and xe2x80x9csolid state electronic devicesxe2x80x9d are being used herein in a comprehensive sense, and are intended to include solid state circuits wherein a complete circuit is formed from a single block or chip of semiconductor material, solid state circuit elements such as Zener diodes, silicone controlled rectifiers, as well as other solid state components such as transistors and diodes. Other devices falling within the comprehensive meaning of the terms include passive components, thermoelectric devices, as well as lasers, each of which typically require contact with a heat exchanger or a thermally conductive path for heat dissipation. These devices are typically incorporated in packages designed for mounting on a chassis in accordance with the individual requirements of the specific circuit. As power and frequency requirements increase, and as the space available for these devices or components shrink, these packages typically require highly efficient, effective, and reliable means for dissipating heat created by the solid state electronic devices during periods of normal operation. Heat is typically transferred by thermal conduction from the package to a mounting surface. This thermal conduction may be undertaken either directly, as occurs when the device is mounted upon the heat dissipating surface, or indirectly as occurs when the device is mounted to a surface which is arranged along a more extensive thermal path leading to a heat dissipating member such as a heat sink or chassis.
In the past, during assembly, it has been common to apply a layer of grease, typically a silicone grease, or a layer of an organic wax to aid in creating a low thermal resistance path between the opposed mating surfaces of the package and the mounting surface. A layer of such grease is typically applied between these mating surfaces in order to displace air and facilitate and enhance thermal conductivity. The devices of the present invention provide significant advantages over the utilization of silicone grease particularly in production and assembly operations.
In accordance with the present invention, therefore, a conformable composite thermally conductive member with a thin surface release layer, e.g., a thin lacquer film thereon is provided for use as an interface for mounting semiconductor devices. This composite, including its release or lacquer surface functions to enhance thermal transfer from the semiconductor or its heat sink to a chassis or the like. When employed in an actual circuit arrangement, assembly time is shortened, and the overall operation is simplified due to the elimination of the need for a liner film. Thereafter, and during normal operation of the semiconductor device, the entire conformable composite member will undergo a phase change, becoming partially molten, thereby enhancing the ability of the overall composite to effectively transfer thermal energy from the device to its mounting chassis. The release layer can be formulated such that it may also melt during operation or remain intact, particularly when the ability to rework the assembly is desired. In addition to being highly thermally conductive, the conformable composites are dimensionally stable, but possess flow characteristics permitting them to respond by fluxing or compressing upon application of only low to modest unit forces and pressures. Because of this property, the conformable composites are highly useful and effective when the delicate nature of the semiconductor package allows exposure to only extremely modest or small external forces. Since some of these products are necessarily exposed to these low or modest forces during assembly, the composites of the present invention will always flow to conform to the configuration of the opposed mating surfaces involved.
Briefly, the composite of the present invention comprises a thermally conductive base or bulk layer of a microcrystalline wax to which a quantity of alumina or other thermally conductive particulate has been added. A thin release layer which may be designated as a lacquer, is applied to the surface in order to form a protective surface or barrier thereon. A variety of hard-non-tacky layers may be found to be useful, including a blend of a silicone and microcrystalline waxes, glassy polyester layer (preferably blended with thermally conductive particulate such as graphite, boron nitride, alumina, and the like)or cross-linked acrylic-based layers. It is preferably compatible with the bulk portion of thermally conductive layer for maximum heat transfer. In use, the lacquer may become merged with or amalgamated into the thermally conductive base layer, with such merger or amalgamation not adversely affecting either the thermal or mechanical properties of the composite, including its thermal stability. If rework properties are desired, the lacquer is formulated such that it retains its integrity while the base layer becomes molten, thus allowing the assembly components to be removed easily.
The term xe2x80x9clacquerxe2x80x9d is used herein in a comprehensive sense, and is intended to define a surface which is somewhat harder than the bulk of the thermally conductive layer which it covers. As set forth above, it is this lacquer layer which performs the function of the anti-blocking liner film traditionally used in the past.
Therefore, it is a primary object of the present invention to provide an improved composite conformable phase change member with an anti-blocking surface, and which is adapted for use as an interface positioned between a semiconductor device and a heat dissipating surface.
It is a further object of the present invention to provide an improved conformable composite member comprising a base layer of high thermal conductivity, together with an integral, compatible outer anti-blocking film or coating thereon to provide surface protection, with the outer film also being highly thermally conductive.
It is a further object of the present invention to provide an improved conformable composite member comprising a base layer of high thermal conductivity, together with an integral, compatible outer anti-blocking film or coating thereon to provide for easy disassembly and reassembly of the electronic package without necessitating the replacement of the phase change pad.
Other and further objects of the present invention will become apparent to those skilled in the art upon a study of the following specification, appended claims, and accompanying drawings.